Apparatus for generating and optically characterizing an aerosol

ABSTRACT

An apparatus having: a vessel for containing a suspension of a liquid and solid particles; a tube having a narrowed portion to draw the suspension from the vessel into the tube when a gas flows through the tube; an aerosol generator coupled to the tube for forming an aerosol from the suspension; a dehydrator coupled to the aerosol generator for removing the liquid from the aerosol forming a dried aerosol; a multiple-pass spectroscopic absorption cell coupled to the dehydrator to pass the dried aerosol into the absorption cell; and a Fourier transform spectrometer coupled to the absorption cell to measure an absorption spectrum of the dried aerosol.

This application claims the benefit of U.S. Provisional Application No.62/918,713, filed on Feb. 16, 2019.

TECHNICAL FIELD

The present disclosure is generally related to aerosol generation.

DESCRIPTION OF RELATED ART

The relationship between aerosol particles and cloud systems is a poorlyunderstood nonlinear process and is the largest uncertainty toaccurately predicting climate and extreme weather events.^(1,2) Aerosolparticles serve as nucleation sites for water molecules to condense intodroplets that can then form into clouds. Recent work posited thataerosol particles from the exhaust of ships enhanced the intensity andelectrification of storms, showing that the density of lightning strikesdoubled over shipping lanes.³ Moreover, ultrafine aerosol particles(diameter <50 nm), once thought to be too small to influence cloudformation, have recently been shown to significantly intensify theconvective strength of cloud systems,² indicating that nanoparticleaerosols may also be used for geoengineering applications.⁴⁻¹⁰

The influence of nanoparticle aerosols on cloud formation is extremelycomplex and hard to disentangle, and a significant need exists toexperimentally model these systems in controlled environments tocarefully examine the nanoscale mechanisms governing these macroscaleprocesses. Aerosols composed of micrometer-sized particles have beenthoroughly investigated for decades.¹¹ However, the experimentalaerosolization and optical detection of nanoparticle aerosols is alongstanding challenge due to factors such as aggregation upon theliquid-gas phase transition, relatively dilute concentrations, or smalllight-matter coupling.¹²

Plasmonic nanoparticles are promising candidates for benchtop aerosolstudies. They couple strongly to light, leading to the capability tooptically detect them in dilute concentrations, and they are alsosensitive to changes in their surrounding environment. A simple harmonicoscillator model can be used to describe the behavior of the plasmonicnanoparticles in an optical field.¹¹ From this model, the imaginaryelectric susceptibility of a plasmonic nanoparticle is χ″=βω_(p) ²ω/[(Lω_(p) ²−ω²)²+β²ω²], where β is the damping constant, L is thedepolarization factor, ω_(p) is the plasma frequency, and ω is thefrequency of the incident light. The imaginary susceptibility, andconsequently the absorption, is a maximum at resonance, ω=√{square rootover (L)}ω_(p), yielding χ″_(max)=ω_(p)/(β√{square root over (L)}).Therefore, a pragmatic nanoparticle to maximize the absorption is a goldnanorod¹³⁻¹⁵ due to its large ω_(p), small L (along the long axis of thenanorod), and mature chemical-based fabrication.

The nanorods will be thermodynamically stable in the gas state when thegravitational force ΔρVg is less than the stabilizing thermal forcesk_(B)T/l, where Δρ is the density difference between the nanorod andgas, V=¾πr²l is the volume of the nanorod, l is the length and r is theradius of the nanorod, k_(B) is the Boltzmann constant, and T is theabsolute temperature. Accordingly, if the length of the gold nanorods issmaller than 3k_(B)T/4πΔρgr²≈μm, then they will remain suspended in thegas state. Gold is also an inert metal, making it biocompatible andenvironmentally friendly. Additionally, recent gram-scale, colloidalgold nanorod synthesis breakthroughs have now made these materialsaccessible in large quantities.¹⁶

BRIEF SUMMARY

Disclosed herein is an apparatus comprising: a vessel for containing asuspension comprising a liquid and solid particles suspended therein; atube having a narrowed portion configured to draw the suspension fromthe vessel into the tube when a gas flows through the tube; an aerosolgenerator coupled to the tube for forming an aerosol from thesuspension; a dehydrator coupled to the aerosol generator for removingthe liquid from the aerosol forming a dried aerosol; a multiple-passspectroscopic absorption cell coupled to the dehydrator to pass thedried aerosol into the absorption cell; and a Fourier transformspectrometer coupled to the absorption cell to measure an absorptionspectrum of the dried aerosol.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation will be readily obtained by reference tothe following Description of the Example Embodiments and theaccompanying drawings.

FIG. 1 schematially illustrates the experimental apparatus to aerosolizeand optically measure gold nanorods in the gas phase in situ.

FIG. 2 shows the geometry of the COMSOL model used in calculatingextinction cross sections. A gold nanorod is built in a box, with oneface as a Perfect Magnetic Conductor (PMC) and another face as a PerfectElectric Conductor (PEC) to account for symmetry. All other edges aresurrounded by Perfectly Matched Layers (PML) to absorb reflections.

FIG. 3 shows the absorbance spectra of high aspect ratio gold nanorodsin the gas phase.

FIG. 4 shows the absorbance spectra of high aspect ratio gold nanorodsin the liquid phase.

FIG. 5 shows a representative TEM image. The scale bar is 500 nm.

FIG. 6 shows aspect ratio statistics corresponding corresponding to FIG.5.

FIG. 7 shows experimental spectra (top) of gold nanorods with aspectratios ranging from 1.5, 2.5, 3, 4.5 in water to 10, 15, 30 in tolueneand three-dimensional finite-element simulation spectra (bottom)matching the experimental parameters and phases.

FIG. 8 shows experimental spectra (top) of gold nanorods with aspectratios of 5, 15, 30 in air and three-dimensional finite-elementsimulation (bottom) matching the experimental parameters and phases.

FIG. 9 shows extinction spectra of a nanorod surrounded by water andair, as the diameter of a water droplet D encasing the nanoroddecreases.

FIG. 10 shows a color map of a nanorod surrounded by water and air, asthe diameter of a water droplet D encasing the nanorod decreases.

FIG. 11 shows the extinction of a gold nanorod as the diameter of thewater droplet decreases.

FIG. 12 shows the scattering of a gold nanorod as the diameter of thewater droplet decreases.

FIG. 13 shows the absorption cross section of a gold nanorod as thediameter of the water droplet decreases.

FIG. 14 shows the evolution of the absorbance peak wavelength as thenanorod aspect ratio and host refractive indexes are varied.

FIG. 15 shows the evolution of the sensitivity as the nanorod aspectratio and host refractive indexes are varied.

FIG. 16 shows the simulated absorbance spectra for plasmonic aerosols asa function of the host gas.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

Disclosed herein is a solution to a decades-old problem ofsimultaneously aerosolizing and measuring the optical response ofplasmonic nanoparticles in the gas phase, thereby uniting the fields ofplasmonics and aerosols. It is shown that the aerosols are opticallyhomogeneous, thermodynamically stable, with wide wavelength tunability,and extremely high sensitivities to their environment that may be usefulin aiding geoengineering challenges. It is anticipated that plasmonicaerosols will open up broad and innovative approaches to understand theunderlying physics of inaccessible climatology, astronomy, petroleum,and medical environments. In the context of vacuummicroelectronics,¹⁷⁻¹⁹ if plasmonic aerosols are encapsulated intomicron-sized elements and gated using external electric fields, then theelectro-optic properties of the element may be reconfigurable bycontrolling the orientational order of the nanorods.^(20,21) Thesematerials may also be useful for nonlinear optics⁶⁻⁸, nanojetprinting²², molecular diagnostics²³, or nanomedicines²⁴.

The disclosed apparatus is illustrated in FIG. 1. It may be used with asuspension of solid particles in a liquid. One example type of particlesis nanorods. The liquid can be any liquid capable of suspending theparticles. Aqueous suspensions are typical. As shown in FIG. 1, thesuspension is placed in a vessel with a tube having a narrowed portion,such as a Venturi tube. When a gas, such as dry air, flows through thetube, it draws the suspension into the tube. The narrowed portionincreases the amount of suspension drawn into the tube.

The suspension then flows into an aerosol generator which forms anaerosol from the suspension. This aerosol contains liquid droplets withsuspended particles. The droplets may be, for example, up to 1 micron indiameter. The aerosol then flows through a dehydrated that removes allor most of the liquid to make a dried aerosol. The dehydrator mayinclude a desiccant the dries the aerosol by diffusion. The driedaerosol may contain only the particles suspended in the gas, or theremay be trace amounts of liquid remaining.

The dried aerosol then flows into a multiple-path spectroscopicabsorption cell, such as a Herriott cell. The optical path length of thecell may be, for example, up to 20 meters long. A Fourier transformspectrometer is then used to measure an absorption spectrum of the driedaerosol. The spectrum may include the IR, visible, and/or UV range,including near and/or far IR.

A vacuum pump at the spectrometer end may be used to draw the gas or airthrough the entire apparatus.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

Materials—Hexadecyltrimethylammonium bromide (CTAB, >98%) and silvernitrate (>99.99%) were purchased from GFS Chemicals.Octadecyltrimethylammonium bromide (OTAB, >98%),benzyldimethylhexadecylammonium chloride (BDAC, >95%), and L-ascorbicacid (>99.0%) were purchased from TCI. Gold (III) chloride trihydrate(99.9%), sodium borohydrate (99.99%), hydroquinone (>99%), and trisodiumcitrate dihydrate were purchased from Sigma-Aldrich. Thiol-terminatedpolystyrene (M_(n)=5000 Da) was purchased from Polymer Source, Inc.

Nanorod synthesis—Multiple nanorod synthesis techniques were used basedon the desired aspect ratio. High aspect ratio nanorods (30-40) weresynthesized according to the procedure described by Kitahata et al.²⁵ ACTAB concentration of 100 mM was used for all high aspect ratiosynthesis methods while the OTAB concentration was varied between 30-75mM to change the nanorod aspect ratio. The temperature was fixed at 20°C. for both the seed and growth solutions. Nanorods with an aspect ratioof approximately 17 were prepared using the seed-mediated proceduredescribed by Zubarev et al.²⁷ Nanorods with an aspect ratio of 5 weresynthesized with a CTAB/BDAC surfactant growth solution as described byPark et al.¹⁶ The resulting gold nanorod suspensions were centrifuged ateither 5,000 rpm (AR 30-40) or 10,000 rpm (AR 5-17) for 15 min. andresuspended in DI water to their initial volumes before anothercentrifugation step and 10-fold concentration. For redispersion intoluene, nanorods were phase transferred using thiol-terminatedpolystyrene according to a previously established procedure.²⁰

Aerosolization—The experimental setup is schematically illustrated inFIG. 1. Aqueous gold nanorod suspensions were aerosolized with anatomizer (TSI, Model 3076) in a non-recirculatory mode that generatedaerosol droplets with an average diameter of 0.3 μm. The flow rate lofthe liquid suspension was 50 mL/h. The wet aerosol was then passedthrough a dehumidifier (TOPAS, DDU 570H) to remove the water content. AHerriott gas cell (Pike Technologies) with a 1-16 m optical path lengthcoupled to a Fourier transform infrared spectrometer (Bruker, Vertex 70)was used for optical measurements, from 0.6-3 μm or 1-10 μm withmilli-optical density (mOD) resolution. The exhaust port from theHerriott cell was connected to a vacuum line at an air flow rate ofapproximately 3.0 L/min. All instruments were connected withstatic-dissipative silicone rubber tubing (McMaster-Carr). During eachsample measurement, aerosols were flowed for 3 min before spectracollection to ensure aerosol saturation in the gas cell. A pure wateraerosol was first collected and subtracted from the nanorod-wateraerosols. Dry air was flowed for 5 min between aerosol samples. Asilicon detector, quartz beamsplitter, and a halogen lamp source wereused for optical measurements in the visible and NIR regions while amercury cadmium telluride (MCT) detector, KBr beamsplitter, and a globarsource were used for measurements in the MIR region.

COMSOL simulations—A gold nanorod was constructed in a box with thelength of the rod in the z-direction and the center of the nanorodlocated at x=y=z=0. The refractive index of the surrounding materialswas considered to be constant for the wavelengths studied (air, water,or toluene, n=1, 1.33, or 1.475, respectively). The refractive index ofgold was taken from Rakić.³² The computational requirements were reducedby symmetry, computing the electric field of only ¼ of the structure.The nanorod and simulation box were cut in half in the xy-plane at z=0and in the xz-plane at y=0. The xy-plane was specified as a PerfectElectric Conductor (PEC) and the xz-plane was specified as a PerfectMagnetic Conductor (PEC). All other edges were surrounded by PerfectlyMatched Layers (PML) to absorb reflections. An illustration of the modelcan be seen in FIG. 2.

A background electric field propagating in the x-direction and polarizedin the z-direction was specified in the calculation to excite theplasmon mode, E_(b,z)=E₀exp(−ik₀n_(med)x), where E_(b,z) is thez-component of the background electric field, E₀=1 V/m, k₀ is the freespace wavevector, and n_(med) is the refractive index of the medium.Absorption cross sections (σ_(abs)) were calculated by integrating thepower dissipation, Q, over the volume of the nanorod,

${\sigma_{abs} = \frac{\int{\int{\int Q}}}{P_{in}}},$where P_(in) is the input power, which is calculated as,

${P_{in} = \frac{E_{0}^{2}}{2Z_{0}n}},$where Z₀ is the characteristic impedance of vacuum. Scattering crosssections (σ_(scat)) were calculated by integrating the Poynting vector,S, over a surface surrounding the simulation domain (the boundarybetween the surrounding medium and the PML),

$\sigma_{scat} = {\frac{\int{\int S}}{P_{in}}.}$Both σ_(abs) and σ_(scat) were multiplied by a factor of four to adjustfor calculating the electric field of only ¼ of the structure.

Results and discussion—FIG. 1 shows a schematic for the transition ofaqueous suspensions of gold nanorods from the liquid phase into the gasphase while simultaneously measuring their optical responses. The goldnanorods were synthesized using wet-seed mediated synthesis approachesthat enabled tuning of the aspect ratio (length l to diameter d) from1.5 to 38.^(16,25,26) Transmission electron microscopy (TEM) was used tomeasure the polydispersity of the nanorods, yielding less than 10% foraspect ratios smaller than 20 and 25% for larger aspect ratios. Toaerosolize the nanorods, a Venturi tube was used to drive high-velocityair over a reservoir of gold nanorods in a liquid suspension, pullingthe suspension into the airstream. Upon exiting the tube, the liquidsuspension breaks apart into an aerosol containing liquid droplets(diameter ˜300 nm) with embedded nanorods. The droplets then enter adehumidifier chamber that evaporates the remaining water from thenanorods, thereby creating a suspension of dry nanorods in the gasphase. To measure the in situ absorbance spectra of the gold nanorods inthe gas phase, a 10-m optical path length Herriott cell was placed atthe exit of the dehumidifier. The Herriott cell was integrated into aFourier transform infrared (FTIR) spectrometer, enabling the opticalsignatures of the plasmonic aerosols to be measured from 0.6 to 3 μm or1 to 10 μm in a continuous manner.

The absorbance spectra of the gold nanorods in the gas phase are shownin FIG. 3, revealing a well-defined absorption peak at 3.3 μm. Therelatively large effective

factor (λ₀/FWHM) of 1.3 in FIG. 3 suggests that the nanorods are notflocculated or aggregated upon transitioning from the liquid to the gasphase. The magnitude of the absorbance peak was constant while themeasurements were performed (average of 16 individual spectra)demonstrating the aerosols are temporally stable. The constantabsorbance peak also implies the aerosols are optically homogeneous,which is expected since the nanorods are generally much smaller than thewavelength of the light (˜λ₀/10). The sharp absorbance peaks around 6 μmare due to the O—H molecular vibration from trace amounts of watervapor. The apparent increase in absorbance near 1 μm may be attributedto these wavelengths being near the wavelength detection limit for thespecific FTIR detector-beam-splitter combination, although triangularplatelet by-products from the high aspect nanorod synthesis may haveresonances in this region as well. The density of nanorods in theHerriott cell is estimated to be ρ=A/σx≈10¹¹ NR/m³, where A=0.18 is thepeak absorbance at 3.3 μm, σ=9.4×10⁻¹⁴ m²/NR is the extinction crosssection of the nanorod, which was retrieved from three-dimensionalfinite-element simulations (COMSOL MULTIPHYSICS, version 5.3a), and x=10m is the path length of light through the Herriott cell.

Due to water's prohibitively large absorption bands beyond 1.2 μm, thenanorods were phase transferred from water into toluene suspensions²⁰ toenable the absorbance spectra in the infrared region to be measured inthe liquid phase, as seen in FIG. 4. While molecular vibrations from thearomatic ring of toluene are still significant throughout the spectrum,leading to detector saturation in some wavelength bands, the absorbancepeak from the nanorods is apparent. TEM images, as shown in FIG. 5, wereused to determine an average aspect ratio of 32.3 (l=646±156 nm, d=20±4nm) as shown in FIG. 6.

The aspect ratio of the gold nanorods was varied from 1.5, 2.5, 3, and4.5 in water to 10, 15, and 30 in toluene in FIG. 7, demonstratingnearly a decade in experimental wavelength tunability from 0.6 to 5 μmin the liquid phase. The effective

factor in the liquid phase varied from 5 to 1 for aspect ratios rangingfrom 1.5 to 30, respectively.

The optical response of gold nanorods in the gas phase with aspectratios of 5, 15, and 30 are shown in FIG. 8, spanning over 2.5 μm inwavelength tunability. The effective

factor in the gas phase varied from 2.4 to 1.3 for aspect ratios of 5 to30, respectively.

Three-dimensional finite-element simulations in FIGS. 7 and 8 were usedto replicate the experimental isotropic spectra of the nanorods in theliquid and gas states. The simulated longitudinal absorbance peakwavelengths are in good agreement with the experimental values as afunction of aspect ratio and solvent index. The effective

factors of the longitudinal absorbance peaks from the simulated spectraare larger than for the experimental spectra due to experimentalvariations in the aspect ratio of the nanorods²⁷ . The small blueshiftedabsorbance peaks, relative to the longitudinal peaks, are due to thetransverse axis of the nanorods.

To understand the spectroscopic evolution of the evaporatingwater-nanorod droplets, further simulations were carried out in FIGS.9-13. Initially, a nanorod (l=60 nm, d=25 nm) was surrounded with water,yielding an extinction peak wavelength at 670 nm (FIG. 9). The nanorodwas then embedded into a water droplet of diameter D=200 nm andsurrounded by air. The magnitude of the peak is increased by ˜⅓ comparedto a homogenous water medium due to the increased scatteringcontribution to the extinction from the water droplet surface. As thediameter of the water droplet decreased to 61 nm, mimicking evaporation,the peak wavelength blueshifted to 624 nm. The extinction peak thendrastically shifted to 582 nm when D=l=60 nm, as seen in FIG. 10, due tothe ends of the nanorod being exposed to air. For droplet diameters lessthan the length of the nanorod the extinction peak remains constant,only slightly increasing in magnitude once all the water is removed.

FIGS. 11-13 show the extinction, scattering, and absorption crosssections of a high aspect ratio gold nanorod (l=450 nm, d=20 nm) as thediameter of the water droplet decreases from D=1000 nm to D=l=450 nm.The most striking feature as the water droplet evaporates is thedecrease in the visible wavelength scattering. The albedo, which is theratio of the scattering cross section to the extinction cross section,is 0.066 for the l=60 nm nanorods and 0.366 for the l=450 nm nanorods.

Transitioning the nanorods from the liquid to the gas state results inlarge shifts in the absorbance peak wavelength, as shown in FIGS. 7-8.By monitoring the peak shifts and comparing the measurements withsimulations, we can accurately determine the local refractive index ofthe medium surrounding the nanorods and therefore infer the state of thenanorod suspension, e.g., water, toluene, or air. Moreover, theeffective

factor is a good metric to ensure the nanorods are not aggregated, asdiscussed with FIGS. 7-8.

To validate that the nanorods are dispersed in air, experiments andsimulations were carried out measuring the absorbance peak wavelength atvarious aspect ratios of the gold nanorods: 7.5, 13, 16.8, 33, and 38 intoluene and air (FIG. 14).

The simulations show that the absorbance peak wavelength dependslinearly on the aspect ratio of the nanorods,λ_(toluene)=0.135(l/d)+0.422 and λ_(air)=0.0883 (l/d)+0.359, and thepeak redshifts as the refractive index of the host medium increases.²⁸

The experimental data were found to agree well with the simulation data,within experimental uncertainty, and that the absorbance peak isproportional to the aspect ratio. Furthermore, the experimental data forthe nanorods in air agree well with the simulation predictions for theirpeak wavelengths, implying the nanorods are homogeneously dispersed asan aerosol.

To further confirm that the nanorods are uniformly suspended in air, thelongitudinal absorption peak wavelength can be related to the refractiveindex of the host medium n_(m) ²⁹ by

$\begin{matrix}{{\lambda = {\lambda_{p}\sqrt{1 + {\left( {1 - L_{}} \right)\left( {n_{s}^{2} - n_{m}^{2}} \right)f} + {\left( {\frac{1}{L_{}} - 1} \right)n_{m}^{2}}}}},} & (1)\end{matrix}$where λ_(p) is the plasma wavelength of gold, n_(s) is the refractiveindex of the ligand shell coating the nanorods, and f is the ellipsoidalvolume fraction of the inner nanorod to the outer ligand shell. Thedepolarization factor of the long axis of the nanorod isL _(∥)=[(1−ϵ²)/ϵ]{[(½ϵ)[ln(1−ϵ)/(1+ϵ)]−1]}where ϵ=√{square root over (1−(l/d)²)}.

If the nanorods are very long (1/L_(∥)>>1) and there is no ligand shell(n_(s)=n_(m)), then Eq. (1) can be differentiated with respect to n_(m)and then series expanded about L_(∥) to approximate the sensitivity,

$\begin{matrix}{\frac{\partial\lambda}{\partial n_{m}} \approx {\frac{\lambda_{p}}{\sqrt{L_{}}}.}} & (2)\end{matrix}$This result implies that if the geometric (L_(∥)) and material (λ_(p))properties of the nanorod are known, the shift in the absorption peakwavelength can be estimated as the host medium surrounding the nanorodsis varied.

In FIG. 14, it is shown that the sensitivity also depends linearly onthe aspect ratio, ∂λ/∂n_(m)=0.098 (l/d)+0.133, for the data retrievedfrom the simulations (solid line). This result is in good agreement withthe experimental data. For smaller aspect ratios, the relationship inEq. (2) is confirmed (dashed line). For larger aspect ratios, thepredictions of Eq. (2) begin to deviate from the simulation data (e.g.,18% at aspect ratio=45), showing the limitations of the simplerelationship. Eq. (2) provides a straightforward means to predict thesensitivity, and the good agreement with experimental and simulationdata further supports the nanorods being thermodynamically stabilized inthe gas phase.

The absorbance peak wavelength shifts are on the order of severalmicrons per refractive index unit (RIU) in FIG. 15. This largesensitivity implies that very small changes in the host medium can bedetected, suggesting that plasmonic aerosols may be good candidates toaccurately probe and model remote environments such as atmosphericsystems at benchtop size scales. To investigate this possibility, goldnanorods with an aspect ratio of 30 were simulated in He, air, and CO₂gaseous environments (FIG. 16). The absorbance peak wavelength shiftedfrom 3275.94 nm for He to 3277.25 nm for CO₂, showing that changes ofΔnm≈10⁻⁴ may be detectable at atmospheric transmission windowwavelengths.^(30,31)

In summary, the aerosolization of gold nanorods from concentrated liquidsuspensions, while simultaneously measuring their optical spectra atbenchtop scales was demonstrated. The plasmonic aerosol absorption peaksare sharp and well defined with effective quality factors as large as2.4. It was shown that by controlling the aspect ratio of the nanorods,the aerosol absorption peaks are broadly tunable over 2500 nm fromvisible to midwave infrared wavelengths. It was found that thesensitivity of the longitudinal absorption peak wavelength to therefractive index of the host medium depends linearly on the nanorodaspect ratio and can be estimated from the geometric and materialproperties of the nanorod. Utilizing this sensitivity dependence, it wasalso shown that minute changes of the host refractive index of 10⁻⁴ maybe detectable, suggesting these materials could be useful forenvironmental or remote sensing.

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a”, “an”, “the”, or “said” is not construed as limitingthe element to the singular.

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What is claimed is:
 1. An apparatus comprising: a vessel for containinga suspension comprising a liquid and solid particles suspended therein;a tube having a narrowed portion; wherein the tube is configured to drawthe suspension from the vessel into the tube when a gas flows throughthe tube; an aerosol generator coupled to the tube for forming anaerosol from the suspension; a dehydrator coupled to the aerosolgenerator for removing the liquid from the aerosol forming a driedaerosol; a multiple-pass spectroscopic absorption cell coupled to thedehydrator to pass the dried aerosol into the absorption cell; and aFourier transform spectrometer coupled to the absorption cell to measurean absorption spectrum of the dried aerosol.
 2. The apparatus of claim1, wherein the aerosol generator is capable of forming aerosol dropletsless than 1 micron in diameter.
 3. The apparatus of claim 1, wherein thedehydrator comprises a desiccant that causes diffusion dehydration ofthe aerosol.
 4. The apparatus of claim 1, wherein the absorption cell isa Herriott cell having an optical path length of up to 20 m.
 5. Theapparatus of claim 1, further comprising: a vacuum pump coupled to theadsorption cell to draw the gas, the suspension, the aerosol, and thedried aerosol through the apparatus.
 6. A method comprising: providingthe apparatus of claim 1; placing the suspension into the vessel;flowing the gas through the tube; and measuring an absorption spectrumof the dried aerosol.
 7. The method of claim 6, wherein the suspensionis an aqueous suspension.
 8. The method of claim 6, wherein the solidparticles are gold nanorods.
 9. The method of claim 6, wherein the gasis dry air.